Batteries store energy in chemical form. A rechargeable battery is a type of battery capable of transmuting electrical charge by storing it in the form of a reversible chemical reaction. When the battery is subsequently placed across a load, this reversible reaction reverses from the direction in the storage mode, thereby producing electrical energy for use by the load.
There are many popular types of rechargeable batteries. Perhaps the most popular are the nickel cadmium and lead acid types. These batteries generally operate over a usual range of ambient temperatures, and store a relatively small amount of charge.
Other types of batteries, which are presently becoming more popular due to relatively high amount of energy storage, are generically labeled as "high temperature batteries". Examples of this kind of batteries are electrochemical storage cells of the alkali metal and chalcogen type, sodium sulfur type, and lithium or lithium fluoride type. Operation of a high temperature battery requires it to be stored in an ambient environment with a temperature of betweeen 300.degree. C. and 500.degree. C. Thus, although these high temperature batteries can store increased amounts of charge, they must be used under difficult operating conditions (very high temperatures). To compound this problem, typically the reliability of these batteries is reduced by any heat cycling of the batteries between ambient temperature and their proper operating temperature. Thus, when using these batteries, continuous reliable operation is highly advantageous. The importance of trouble free operation is made doubly evident when it is considered that even if servicing of these batteries is desired, the process requires a significant amount of cooling time before the parts of the batteries would be cool enough to be handled by service personnel. During this cooling time, the batteries will necessarily be out of service.
Single battery cells are generally available in the range of approximately a half a volt to five volts, with the specific voltage of the battery cell depending on properties of the chemical reaction which is occurring within the battery cell. Thus, to obtain a battery which has a higher voltage than this relatively low cell voltage, typically a plurality of cells are placed in series, to thereby add the respective cell voltages to obtain a resultant higher voltage. Sodium sulfur batteries, for example, have a cell voltage of approximately two volts. Thus, should a battery voltage of 48 volts be required, 24 of the cells would be required to be connected in series. However, this connection in series presents some problems with respect to operational reliability and to maximum battery efficiency.
One such problem is due to the fact that when a battery cell fails it will typically fail into the open circuit state. This failure into open circuit state would effectively destroy the utilitarianism of the entire battery. Thus, in the example given above of a 48-volt battery, a single cell of the 24 cells in the battery malfunctioning into an open circuit state would necessitate the replacement or repair of the entire battery. As such, it would be advantageous in the art to have a device which obviates this problem.
An early attempt at such a device is described in U.S. Pat. No. 2,624,033. This patent teaches placing individual diodes in parallel across each series connected cell. These diodes are placed so that normally a charged cell would reverse bias these diodes. However, when a cell either open circuits or is discharged close enough to zero volts, the related diode shunt across the particular cell will be forward biased. Thereby the particular cell will be effectively shorted out. The patented system has the disadvantage that in order for the diode to shunt across the particular cell, the diode must be forward biased and operating correctly. There is no permanent state change in the diode, and thus a failure of the diode would cause the battery to malfunction.
Another proposed solution to the problem is suggested in the disclosure of U.S. Pat. No. 3,102,222. This patent teaches a device which is specialized to high temperature catalytic battery cells, whereby by sensing the temperature of a particular battery cell, the condition of that particular cell can be approximated according to a predetermined algorithm. A switch 1 is normally closed and connected in series between the battery and the charging unit. The switch 1 is arranged to open when the temperature of the catalyst used in the battery reaches a predetermined value. The patent does not teach a method of shunting across individual cells in response to cell failure. Furthermore, this technique would only be applicable to high temperature battery cells.
A further proposed solution to the problem is taught in U.S. Pat. No. 4,303,877, the disclosure of which is expressly incorporated herein by reference. This patent teaches a plurality of battery cells of the electrochemical storage type in series. Shunted across each such cell is a temperature sensitive switch and a diode in series with heating device. In one preferred embodiment, when a cell fails into the open circuit state, the diode is forward biased thereby energizing the heating element. This heating e1ement then heats the temperature sensitive switch, which permanently changes position--similar to a fusible link. This temperature sensitive switch thus permanently changes position in response to a cell of the battery failing. The failed cell is thereby effectively shorted across. While the general technique used is extremely effective, a disadvantage exists in the relative complexity and impracticality of the many components being used within a high temperature battery. The present invention overcomes all these problems by a single component performing all these functions as described herein.
Another problem which tends to lower the reliability of batteries is uneven cell charging. For reasons known to those skilled in the art, some battery cells will require and/or give up their charge at a more rapid rate than other cells. Also, different batteries have uneven leakage currents, thereby yielding a variable shelf life. Thus, at any given time of charge in the prior art typical charging apparatus, all battery cells would not be at the same charge level. However, it is particularly advantageous to charge all battery cells to substantially the same charge--that same charge being their full possible charge. However, a countervailing consideration is that there are many problems associated with overcharging a battery cell. For instance, in the nickel cadmium and lead acid type of battery cells overcharging leads to a phenomenon known as gasing. Once an electrode of a battery has been fully reconstituted, charging beyond that point causes gases to be liberated by the electrodes at the expense of the electrolytes. In open or vented cells, these gases can escape, although some damage to the electrode will be caused as a result. However, a hermetically sealed battery suffers from a more acute problem as the gas pressure will build up in the battery, which could conceivably lead to an explosion. In all types of batteries, overcharging will cause at least excessive heating of the battery cell, as the extra charge which cannot be converted in to chemical energy is dissipated as heat. Thus, there is a need for a device which will allow batteries to reach their substantially full charge--meaning full charge on every cell within the battery--while not causing overcharge of the battery cells with the particular disadvantages associated therewith.
A particular proposed solution to this problem is discussed in Canadian Pat. No. 698,137. This patent teaches a plurality of diodes, each being connected in parallel across a cell of a battery in a forward biased direction. These diodes are specially constructed so that the forward voltage drop across the diode (necessary to forward bias it) is slightly greater than the cell voltage of the battery. Thus, when the battery is being charged, the diode shunting each particular cell will become conducting at such time as the battery cell voltage reaches a voltage high enough to forward bias the diode.
Another proposed solution is taught by U.S. Pat. No. 3,343,058. This patent teach using a tunnel diode device suitably shunted across each battery cell. When the battery reaches a breakdown voltage of the tunnel diode, the tunnel diode begins conducting, thus limiting the cell voltage similar to the diode in the Canadian patent.
However, the problem with both of these later two proposals is that there will be a necessary trade-off between charging current and the size of the semiconductor device being used. For any reasonable charging current the size of diode would have to be large enough to dissipate that entire amount of current, so that when a cell becomes fully charged the associated diode can conduct the remainder of the current around the fully charged cell. This would necessitate physically large components for any reasonable charging rate.
Another proposed solution to the problem involves using some kind of intelligent unit to sense the charge on a battery cell and having this unit cause switching units to switch around the particular battery cell according to a predetermined algorithm. This method is exemplified by U.S. Pat. No. 4,061,955 and U.S. Pat. No. 4,238,721. Both of these patents teach extremely complicated systems being used to monitor the cell voltage and a analog switch device capable of carrying the proper amount of current shunting around this cell at the proper time.
In stark contrast, according to the present invention, this shunting is done by a specially constructed zener diode. This operation of the present invention is extremely advantageous in high temperature battery cells, such as sodium sulfur. A sodium sulfur battery cell can be overcharged to a certain extent without significant deleterious effects occurring thereto. Also, the ambient temperature of the cell is already 350.degree. C., the heating effects added by overcharging the cell can be negligible. However, although the present invention finds a great usefulness in high temperature batteries such as sodium sulfur, it is not intended to be limited to these kinds of batteries and would find many applications in low temperature batteries such as nickel cadmium and lead acid as well. Since the requirements of a high temperature battery make maintenance of the battery difficult, these devices are particularly cost justified in these high temperature batteries.
Thus it is an object of the present invention to overcome the problems stated above, by use of a specially constructed zener diode looping element for use in a battery with a plurality of cells, the looping element being used as a shunt across each said cell, which includes a zener diode p-n junction, packaged in a heavy duty package so that failure of the package will not occur during extreme overcurrent conditions. The zener diode fuses to a permanent short circuit condition when a predetermined amount of current is passed through the zener diode in a forward biased direction for a predetermined length of time.
A battery according the present invention includes: a plurality of battery cells connected in serial with a plurality of zener diode looping elements, each connected in parallel across each cell, each of the zener diode elements having its cathode connected to the anode of each cell and each zener diode having its anode connected to the cathode of each cell. The looping element is constructed so that when a predetermined amount of current is passed through it in a forward direction the zener diode fuses permanently into a short circuit state. The battery system can alternatively include terminals for relaying a voltage from the battery to a load; a plurality of battery cells connected in series betweens the terminals; a plurality of zener diodes, each zener diode being connected in parallel across one of the battery cells, so that the zener diode is normally reverse biased; and a charger for applying charge to the battery according to a predetermined program; the predetermined program ensuring that when the battery voltage is high enough to cause reverse breakdown of the zener diodes that the current emitted from the charger will not cause destruction of said zener diodes.